The characterization of an isolate of swine infertility and respiratory syndrome (SIRS) virus (ATCC VR-2332) is reported. A commercial cell line (CL2621) was used for the propagation of the virus for all assays. Laboratory studies indicate that this isolate is a fastidious, nonhemagglutinating, enveloped RNA virus. Cesium chloride-purified virions visualized by electron microscopy were spherical particles with an average diameter of 62 nm (range: 48-83 nm) and a 25-30 nm core surrounded by an envelope. Virus replication was restricted to the cytoplasm, as demonstrated by immunofluorescence. The virus did not react serologically with antisera to several common porcine viruses or with antisera to known viruses in the alphavirus, rubivirus, pestivirus, and ungrouped lactic dehydrogenase virus genera of the Togaviridae. However, convalescent sow sera and rabbit hyperimmune sera neutralized the SIRS virus at titers of 1:256 and 1:512, respectively. The virus was stable at 4 and -70 C, but was labile at 37 and 56 C. The properties of this isolate of SIRS virus resemble those of the family Togaviridae but do not match the described genera.